A Brief Overview of Snake Evolution

Among reptiles, snakes have long attracted attention because of their distinctive body plan and physiological traits. Their unusual form has also made them powerful symbols in many human cultures. Today, more than 4,000 living species of snakes have been identified. They belong to the suborder Serpentes within the order Squamata (the scaled reptiles) and account for roughly 35% of all squamate species.

In addition to snakes, Squamata also includes several other major groups such as dibamids (Dibamia), geckos (Gekkota), lacertoids (Laterata), skinks and their relatives (Scincomorpha), iguanians (Iguania), and anguimorph lizards (Anguimorpha). The famous marine reptiles known as mosasaurs (Mosasauria) from the Cretaceous are currently classified within Anguimorpha as well.

Phylogenetic tree of Squamata（圖片來源：F-L Reptile，採用 CC BY-SA 4.0 授權）

Compared with other squamates, snakes display an extraordinary diversity of species and ecological roles. Today they can be found in forests, deserts, grasslands, and oceans across the world. This success may be attributed to their elongated bodies, highly flexible skeletons, powerful constriction abilities, venom systems, and extremely mobile skulls, all of which give them advantages in a wide range of ecological niches. Their evolutionary success is also reflected in two major adaptive radiations: one that occurred roughly 150 million years ago during the Late Jurassic to Early Cretaceous, and another that followed the end-Cretaceous mass extinction.

Because snakes are descendants of four-limbed tetrapods, the evolutionary loss of their limbs has become a subject of intense interest for evolutionary biologists. Unfortunately, snake fossils from before the Cretaceous are extremely rare or fragmentary. As a result, the early evolutionary history of snakes remains obscured by considerable uncertainty. Most fossils that can be confidently identified as true snakes come from the mid-to-late Cretaceous and later.

Furthermore, snakes are not the only squamates that evolved limbless forms. Other groups also include species that have independently lost their limbs, such as dibamids, amphisbaenians within the lacertoid lineage, certain limbless skinks, and anguids. These lineages each evolved limblessness independently at different times in evolutionary history. Because of this repeated evolution of snake-like body plans, fossil classification based solely on morphology can be extremely challenging. More comprehensive data are therefore necessary to clarify snake evolution.

（圖片來源：Rafe M. Brown、Tiutenko，採用 CC BY 4.0 授權；圖片來源：Diogo B. Provete、Franz Xaver，採用 CC BY-SA 2.5/CC BY-SA 3.0授權）

The origin of snakes remains one of the most controversial questions in vertebrate evolution. Researchers from many disciplines—including paleontology, ecology, embryology, and phylogenetics—have attempted to resolve this mystery. Even today, however, no definitive answer exists. Some researchers have even proposed the non-mainstream hypothesis that snakes evolved from mosasaurs.

According to recent phylogenetic studies of squamates, snakes appear to be closely related to iguanians and anguimorph lizards. These three groups together form a clade known as Toxicofera, meaning “the venom bearers.” Researchers suggest that the ability to produce venom may have originated as a shared ancestral trait within this clade. In this view, species that are considered non-venomous today may actually represent lineages that lost their venom secondarily during evolution. Some of them may even retain small amounts of venom that are typically overlooked.

Various hypotheses have been proposed regarding the lifestyle of the earliest snake ancestors. These include the aquatic hypothesis, the terrestrial hypothesis, and the burrowing hypothesis. Both aquatic and burrowing lifestyles could potentially drive evolutionary adaptations such as limb reduction and the development of elongated, streamlined bodies.

However, studies of well-preserved skull fossils from the Late Cretaceous snakes Dinilysia patagonica and Najash rionegrina have provided new insights. By reconstructing the brain structure of these ancient snakes and comparing it with those of modern squamates through phylogenetic analysis, researchers have suggested that Mesozoic snakes were more likely associated with a burrowing lifestyle. Nevertheless, they were probably not strictly subterranean specialists. Instead, they may have been opportunistic animals capable of adapting to a variety of environments.

Dinilysia patagonica（圖片來源：Nobu Tamura，採用 CC BY-SA 3.0 授權）

Najash rionegrina（感謝Raúl O. Gómez提供）

Although the exact stem group of snakes remains unclear, several fragmentary fossils may represent early snake-like reptiles. Whether these animals truly belong within Serpentes is still debated. Examples include Eophis underwoodi, Portugalophis lignites, Parviraptor estesi, and Diablophis gilmorei. All four species date to the Jurassic Period. Their known fossils consist mainly of fragmentary jaw bones, and their classification has largely been based on tooth morphology, which introduces a certain degree of uncertainty.

Reconstruction of an ancestral snake（感謝Julius T. Csotonyi提供）

Among them, Eophis underwoodi is currently considered the earliest known potential snake fossil. Based on the size of its jaw fragments, it is estimated to have been a small animal about 25 centimeters long. The fossil dates to the Bathonian stage of the Middle Jurassic and was discovered in the Kirtlington Quarry in Oxfordshire, England. Initially it was interpreted as a snake-like lizard belonging to Anguimorpha, but since 2015 it has been reclassified as a member of the snake lineage.

Researchers infer that these Jurassic snake-like animals probably still possessed limbs, although the limbs had already become quite small. Fossils from the Early Cretaceous provide further clues. One well-known example is Tetrapodophis amplectus, which possessed four limbs that were extremely reduced in size. Whether Tetrapodophis truly belongs to Serpentes remains controversial. At one point it was even classified among mosasaurs within Anguimorpha before later being reassigned to the snake lineage, leaving its position uncertain.

By the Late Cretaceous, several fossils can be confidently identified as true snakes. Species such as Najash rionegrina, Pachyrhachis problematicus, and Haasiophis terrasanctus all possessed small hind limbs, while their forelimbs had already disappeared.

Tetrapodophis amplectus fossil（圖片來源：Ghedoghedo，採用 CC BY-SA 4.0 授權）

Reconstruction of Tetrapodophis amplectus（感謝Julius T. Cstonyi提供）

If evolutionary relationships are reconstructed solely from molecular phylogenetic studies of living squamates, snakes appear to have diverged from their closest relatives—iguanians and anguimorphs—during the Late Triassic. This suggests that the earliest members of the snake lineage may have originated even earlier than the fossil record currently indicates.

What we can say with reasonable confidence today is that snakes evolved from venomous lizard ancestors belonging to Toxicofera. Evidence also suggests that these ancestral lizards may have been at least partially burrowing animals rather than marine reptiles derived from sea-dwelling lizards. Nevertheless, the evolutionary origin of snakes remains enigmatic. Only the discovery of more complete fossils representing early stem snakes will allow researchers to resolve this long-standing evolutionary mystery.

Reconstruction of an early snake（感謝Julius T. Csotonyi提供）

Author: Shui-Ye You

Reference:

1. Caldwell MW et al. (2015). The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature Communication.

2. Grower DJ. (2022). The origin and early evolutionary history of snakes. Cambridge University Press.

3. Macrì S et al. (2023). Reconstructing the origin and early evolution of the snake brain. Sci Adv.

4. Title PO et al. (2024). The macroevolutionary singularity of snakes. Science.

5. Whiteside DI et al. (2022). A Triassic crown squamate. Sci Adv.